This invention relates to a three-way valve for changing over the flow of high-temperature gas containing corrosive components and/or particulate matter, such as exhaust gas from an engine, and an engine exhaust gas purifier having such a valve.
Emissions from a diesel engine contain various components that can cause air pollution. One of such components is combustible particulate material (hereinafter called PM) mainly comprising carbon. Various improvements on engines have of course been proposed to remove PM. But as means for removing PM more reliably, an exhaust purifier having a trapper (as disclosed in unexamined Japanese patent publication 58-51235) mounted in the exhaust line of an engine to trap PM is proposed. Some of such trappers are actually already used in commercial cars.
Trappers need periodical regeneration in which trapped PM is removed by burning. It is difficult to burn PM while exhaust is being introduced from the engine. But stopping the engine for regeneration is impractical. Thus, it is an ordinary practice to bifurcate the exhaust line and mount two trappers in the respective bifurcated paths of the exhaust line so that either one of the trappers can be regenerated while exhaust is being directed to the other trapper.
This arrangement requires a valve capable of selectively closing one of the bifurcated paths of the exhaust line in which the trapper is to be regenerated, while directing exhaust only into the other bifurcated path. For such a valve, an ordinary on-off valve or changeover valve is useless because engine exhaust is a high-temperature corrosive gas and no lubricant such as lubricating oil cannot be used in such a gas.
To solve this problem, unexamined Japanese patent publication 56-92318 proposes a particulate trapping device having a flap valve provided in the exhaust line at the branch point of the two bifurcated paths, the flap valve selectively closing one of the bifurcated paths by pivoting about a pivot axis.
Unexamined Japanese patent publication 7-224633 discloses an exhaust purifier having a three-way valve capable of changing over the flow of exhaust gas. As shown in FIG. 6, this three-way valve comprises an arm 33 pivotable about a pivot axis provided at the branch point of two bifurcated paths 31, 32, and two valve bodies 35 having spherical sealing surfaces 34 facing away from each other. By pivoting the arm 33 in either direction, one of the valve bodies 35 abuts the edges of the opening of one of the bifurcated paths.
These conventional valves can quickly open and close the openings of the bifurcated paths. But since the path in which the trapper to be regenerated is mounted is closed, PM may be burned incompletely due to shortage of oxygen. To prevent incomplete burning, unexamined Japanese patent publication 56-92318 proposes to form orifices in the flap valve so that part of exhaust gas flows into the path in which the trapper is being regenerated. For the same purpose, unexamined Japanese patent publication 7-224633 proposes to provide air supply means to forcibly supply air from outside into the bifurcated path in which the trapper is being regenerated.
The device disclosed in unexamined Japanese patent publication 56-92318 has a problem in that seal between the flap valve and the trapper case seal is insufficient. Due to insufficient seal, exhaust gas will flow not only through the orifices but through the sealed portions into the bifurcated path in which the trapper is being regenerated. This makes the control of oxygen supply difficult. Since the oxygen supply is unstable, PM is burned unstably, so that the trapper tends to be regenerated unstably and unevenly.
In the device disclosed in unexamined Japanese patent publication 7-224633, the passage is closed by bringing a spherical sealing surface into line contact with the edge of the inlet of the bifurcated path. Thus, the surface pressure at the sealing portion is large. This increases the possibility of damaging the sealing surface of the valve body and the edge of the mating member. Also, because of line contact, the sealing properties tend to deteriorate due to inaccurate finish of the spherical sealing surface, corrosion of the sealing surface by exhaust gas, or floating of the sealing surface due to accumulation of PM.
An object of this invention is to provide a three-way valve which is free of such problems of prior art devices and which is high in sealing reliability, and to provide an exhaust purifier in which is used this valve.